The present invention relates to a pressure regulating valve for hydraulic brake systems of vehicles comprising a housing including a pressure chamber communicating with a master brake cylinder through an inlet and with at least one wheel brake actuating cylinder through an outlet, a differential piston arranged in the pressure chamber and acted upon towards the outlet by a spring, the piston's larger surface being exposed to the outlet pressure while its smaller surface is exposed to the inlet pressure and an elastomeric sealing ring arranged in the pressure chamber between inlet and outlet and bounding a first passageway which extends from the inlet to the outlet side of the chamber and is adapted to be closed by axial displacement of the differential piston on attainment of a pressure in the inlet and outlet which is predetermined by the differential piston surfaces and the spring, the sealing ring including on its side close to the inlet a sealing lip cooperating with the wall of the pressure chamber and closing a second passageway in the event of a pressure gradient from the inlet to the outlet and opening it in the event of a pressure gradient in the opposite direction.
Such pressure regulating valves are preferably inserted in the brake line between the master brake cylinder and the brake actuating cylinder of the rear wheels in order to reduce in higher brake pressure ranges the brake pressure of the rear wheel brakes relative to that of the front wheel brakes to take into account the weight transfer of the vehicle.
In a known pressure regulating valve of this type, such as disclosed in German Patent DE-OS No. 1,580,148, the sealing ring arrangement includes an integrally formed elastomeric sealing ring which, in the valve's inactive position, has its end surface close to the outlet side in abutting engagement with a shoulder formed in the pressure chamber while its end surface close to the inlet side is in abutting engagement with a differential piston collar which recedes radially relative to the sealing lip, in which fluid passageways are provided between the collar and the associated end surface of the sealing ring, and in which further the differential piston includes a cylindrical valve head which in the presence of an axial movement of the differential piston towards the inlet side plunges into the bore of the sealing ring in a sealed and sliding relationship thereto, thereby closing the first passageway. Because the sealing ring is completely surrounded by brake fluid, its construction becomes complicated and expensive. The sealing ring is not held in a well-defined position in the valve housing, it is allowed to follow the movement of the differential piston in the closing direction when the latter is actuated, so that the change-over point of the valve is not clearly defined. This is particularly critical when the brake system is actuated slowly. The consistently sliding and sealing plunging movement of the valve head arranged at the differential piston into the inner passageway of the sealing ring may cause grooving of the sealing ring in this area and eventually lead to a failure of the valve function.